Communicating method between IPv4 terminal and IPv6 terminal and IPv4-IPv6 converting apparatus

ABSTRACT

In an apparatus, there are provided an IP transmitting and receiving unit for transmitting and receiving an IPv4 packet and an IPv6 packet; an IP header converting unit for performing a mutual conversion of the IPv4 packet and the IPv6 packet by an IP header conversion; a DNS substituting unit for receiving a domain information capturing request sent from an IPv4 terminal or an IPv6 terminal and substituting its process; an IPv4 address capturing unit for capturing an IPv4 address from a DHCP server; and an IP address conversion information holding unit for holding an IPv6 address of the IPv6 terminal and the IPv4 address captured by the IPv4 address capturing unit in correspondence to each other. Thus, a communication with the IPv4 terminal can be executed without preliminarily fixedly allocating the IPv4 address to the IPv6 terminal.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a communicating method betweenan IPv4 terminal and an IPv6 terminal and to an IPv4-IPv6 convertingapparatus. More particularly, the invention relates to method andapparatus for realizing a communication between an IPv4 terminal usingIPv4 (Internet Protocol version 4) as a communication protocol and anIPv6 terminal using IPv6 (Internet Protocol version 6) as acommunication protocol.

[0002] As a method of realizing a communication between an IPv4 terminaland an IPv6 terminal, there is a method of using an ‘IPv4-mapped IPv6address’ and an ‘IPv4-compatible IPv6 address’ described in RFC (RequestFor Comments) 1884 issued by IETF (Internet Engineering Task Force).

[0003] The ‘IPv4-mapped IPv6 address’ is an IPv6 address in which “0” isset to the 127th to 48th bits among 128 bits of the IPv6 address, “1” isset to the 47th to 32nd bits, and an ‘IPv4 address’ is set to the 31stto 0th bits.

[0004] The ‘IPv4-compatible IPv6 address’ is an IPv6 address in which“0” is set to the 127th to 32nd bits among 128 bits of the IPv6 addressand the ‘IPv4 address’ is set to the 31st to 0th bits.

[0005] When the communication between the IPv4 terminal and the IPv6terminal is executed, an IPv4 address is preliminarily fixedly allocatedto the IPv6 terminal which communicates with the IPv4 terminal. A packetconverting apparatus is provided on the way of a path through which theIPv4 terminal and the IPv6 terminal communicate and a mutual conversionof IP headers of an IPv4 packet (RFC791) and an IPv6 packet (RFC1883) isperformed.

[0006] In an IPv4 network, the IPv4 address of the IPv4 terminal and theIPv4 address allocated to the IPv6 terminal are used and a communicationis executed by using the IPv4 packet. In an IPv6 network, the‘IPv4-mapped IPv6 address’ in which the ‘IPv4 address of the IPv4terminal’ has been set is used as an IPv6 address of the IPv4 terminal.In the IPv6 network, the ‘IPv4-compatible IPv6 address’ in which the‘IPv4 address allocated to the IPv6 terminal’ has been set is used as anIP address of the IPv6 terminal and a communication is executed by usingthe IPv6 packet.

[0007] The packet converting apparatus converts the ‘IPv4-mapped IPv6address’ included in the IPv6 packet into the ‘IPv4 address of the IPv4terminal’ and converts the ‘IPv4-compatible IPv6 address’ to the ‘IPv4address allocated to the IPv6 terminal’, thereby converting the IPv6packet into the IPv4 packet. Contrarily, the ‘IPv4 address of the IPv4terminal’ included in the IPv4 packet is converted into the ‘IPv4-mappedIPv6 address’ and the ‘IPv4 address allocated to the IPv6 terminal’ isconverted into the ‘IPv4-compatible IPv6 address’, thereby convertingthe IPv4 packet into the IPv6 packet. Consequently, the communicationbetween the IPv4 terminal and the IPv6 terminal can be executed.

[0008] As a method of realizing the communication between the IPv4terminal and the IPv6 terminal, IETF also proposed a method called adual stack. According to the method, the IPv6 terminal has both of thecommunication protocols of IPv4 and IPv6. A communication is executedbetween the IPv6 terminals by using the IPv6 protocol. A communicationis executed between the IPv4 terminals by using the IPv4 protocol. Thus,the communication between the IPv4 terminal and the IPv6 terminal can beexecuted.

[0009] IETF also proposed a method called an IP tunneling. This is amethod such that when the IPv4 network exists on a communication pathbetween two IPv6 terminals and a communication cannot be executeddirectly by the IPv6 packet, the IPv6 packet is encapsulated by the IPv4header and is allowed to pass through the IPv4 network. Similarly, whenthe IPv6 network exists on a communication path between the IPv4terminals and a communication cannot be executed directly by the IPv4packet, the IPv4 packet is encapsulated by the IPv6 header and isallowed to pass through the IPv6 network. Consequently, even when thereis the IPv4 network on the communication path, the communication betweenthe IPv6 terminals can be executed. Even when there is the IPv6 networkon the communication path, the communication between the IPv4 terminalscan be executed.

[0010] According to the method, the communication between the IPv4terminal and the IPv6 terminal can be realized by a simple operation ofadding a fixed pattern of 96 bits to the IPv4 address or deleting thefixed pattern of 96 bits from the IPv6 address.

[0011] According to the method called a dual stack, by selectively usingthe communication protocols of IPv4 and IPv6 in accordance with acommunication partner, the communication between the IPv4 terminal andthe IPv6 terminal can be realized.

[0012] According to the method called an IP tunneling, by encapsulatingthe packet by the header of the relevant communication protocol andpassing the resultant data through the network existing on thecommunication path between the two terminals, the communication betweenthe two terminals can be realized.

SUMMARY OF THE INVENTION

[0013] The convention techniques, however, have the following problems.

[0014] A first problem is that the IPv4 address has to be fixedlyallocated to all of the IPv6 terminals which have a possibility ofcommunication with the IPv4 terminal. This further consumes the IPv4address which is short and accelerates its shortage.

[0015] A second problem is that since mutual domain information (forexample, an IP address corresponding to a domain name) cannot bereferred to between the IPv4 network and the IPv6 network, the partnercan be designated not by the domain name but by only an actual IPaddress. This results in that the communication partners have to bemutually aware whether the partner uses IPv4 or IPv6 as a communicationprotocol.

[0016] A third problem is that, in the method called a dual stack, theIPv4 protocol is provided for all of the IPv6 terminals having thepossibility of communication with the IPv4 terminal and the IPv4 addresshas to be fixedly allocated. This results in that a larger memorycapacity is necessary as compared with that in case of providing onlythe IPv6 protocol and, in a manner similar to the first problem, theIPv4 address which is short is further consumed and the shortage isaccelerated.

[0017] A fourth problem is that, in the method called an IP tunneling,when the IPv4 network exists on the communication path between the twoIPv6 terminals, in order to pass the IPv6 packet by encapsulating it bythe IPv4 header, a setting for it has to be preliminarily performed.Similarly, when the IPv6 network exists on the communication pathbetween the two IPv4 terminals, in order to pass the IPv4 packet byencapsulating it by the IPv6 header, a setting for it has to bepreliminarily performed. The method can be used only between theterminals in which the setting of the IP tunneling has preliminarilybeen performed. The communication cannot be executed between theterminals in which the setting is not preliminarily performed.

[0018] It is, therefore, an object of the invention to provide methodand apparatus in which only an IPv6 protocol is provided for an IPv6terminal, even if an IPv4 address is not preliminarily fixedlyallocated, a communication between an IPv4 terminal and the IPv6terminal can be executed, a partner is designated not by an IP addressbut by a domain name, and the communication between the IPv4 terminaland the IPv6 terminal can be executed.

[0019] According to the first aspect, in a communication network systemcomprising an IPv4 network, an IPv6 network; and an IPv4-IPv6 convertingapparatus connected to both of them, an IPv4 terminal of the IPv4network inquires an IP address for a domain name of an IPv6 terminal ofthe IPv6 network to the IPv4-IPv6 converting apparatus. Thus, theIPv4-IPv6 converting apparatus obtains the IPv6 address corresponding tothe domain name from a DNS (Domain Name System) v6 server apparatus formanaging domain information of the IPv6 network, dynamically acquiresthe IPv4 address corresponding to the IPv6 address from a DHCP (DynamicHost Configuration Protocol) v4 server apparatus for dynamicallyallocating the IPv4 address, and notifies to the IPv4 terminal. The IPv4terminal forms an IPv4 packet by setting an own IPv4 address to an IPv4source address and by setting the notified IPv4 address to an IPv4destination address and sends the IPv4 packet to the IPv4-IPv6converting apparatus. The IPv4-IPv6 converting apparatus converts theIPv4 source address included in the received IPv4 packet into an IPv6source address by adding fixed data to the IPv4 source address andconverts an IPv4 destination address included in the IPv4 packet intothe IPv6 address corresponding to the IPv4 destination address, therebyobtaining an IPv6 destination address, forming an IPv6 packet from theIPv4 packet, and sending the IPv6 packet to the IPv6 network. The IPv6terminal forms the IPv6 packet by setting the own IPv6 address to theIPv6 source address and setting the IPv6 source address included in thereceived IPv6 packet to the IPv6 destination address, and transmits theIPv6 packet to the IPv4-IPv6 converting apparatus. The IPv4-IPv6converting apparatus deletes the fixed data from the IPv6 destinationaddress included in the received IPv6 packet, converts the resultantIPv6 destination address to the IPv4 destination address, and convertsthe IPv6 source address included in the IPv6 packet to the IPv4 addresscorresponding to the IPv6 source address, thereby obtaining the IPv4source address, forming the IPv4 packet from the IPv6 packet, andtransmitting the IPv4 packet to the IPv4 network.

[0020] In the communicating method between the IPv4 terminal and theIPv6 terminal according to the first aspect, when the communication fromthe IPv4 terminal side of the IPv4 network to the IPv6 terminal of theIPv6 network is started, the IPv4 address is dynamically allocated tothe IPv6 terminal of the IPv6 network. Consequently, it is unnecessaryto preliminarily fixedly allocate the IPv4 address to the IPv6 terminal,so that the consumption of the IPv4 address can be suppressed. Since theIPv4-IPv6 converting apparatus automatically obtains the IP address forthe domain name of the IPv6 terminal from the DNSv6 server apparatus,the IPv4 terminal can designate the domain name and communicate with theIPv6 terminal.

[0021] According to the second aspect, in a communication network systemcomprising an IPv4 network, an IPv6 network, and an IPv4-IPv6 convertingapparatus connected to both of them, an IPv6 terminal of the IPv6network inquires an IP address for a domain name of an IPv4 terminal ofthe IPv4 network to the IPv4-IPv6 converting apparatus. Thus, theIPv4-IPv6 converting apparatus acquires an IPv4 address corresponding tothe domain name from a DNSv4 server apparatus for managing domaininformation of the IPv4 network, converts the IPv4 address to the IPv6address by adding fixed data to the IPv4 address, and notifies the IPv6terminal of the IPv6 address. The IPv6 terminal forms an IPv6 packet bysetting an own IPv6 address to an IPv6 source address and setting thenotified IPv6 address to an IPv6 destination address and transmits theIPv6 packet to the IPv4-IPv6 converting apparatus. The IPv4-IPv6converting apparatus deletes the fixed data from the IPv6 destinationaddress included in the received IPv6 packet and converts the resultantIPv6 destination address to an IPv4 destination address, dynamicallyacquires the IPv4 address corresponding to the IPv6 source addressincluded in the IPv6 packet from a DHCPv4 server apparatus fordynamically allocating the IPv4 address, forms the IPv4 packet from theIPv6 packet by using the IPv4 address, and transmits the IPv4 packet tothe IPv4 network. The IPv4 terminal forms the IPv4 packet by setting theown IPv4 address to an IPv4 source address and setting the IPv4 sourceaddress included in the received IPv4 packet to an IPv4 destinationaddress and transmits the IPv4 packet to the IPv4-IPv6 convertingapparatus. The IPv4-IPv6 converting apparatus converts the IPv4 sourceaddress to the IPv6 source address by adding fixed data to the IPv4source address, obtains the IPv6 destination address by converting theIPv4 destination address included in the IPv4 packet to the IPv6 addresscorresponding to the IPv4 destination address, forms the IPv6 packetfrom the IPv4 packet, and transmits the IPv6 packet to the IPv6 network.

[0022] In the communicating method between the IPv4 terminal and theIPv6 terminal according to the second aspect, when the communicationfrom the IPv6 terminal side of the IPv6 network to the IPv4 terminal ofthe IPv4 network is started, the IPv4 address is dynamically allocatedto the IPv6 terminal of the IPv6 network. Consequently, it isunnecessary to preliminarily fixedly allocate the IPv4 address to theIPv6 terminal, so that the consumption of the IPv4 address can besuppressed. Since the IPv4-IPv6 converting apparatus automaticallyobtains the IP address for the domain name of the IPv4 terminal from theDNSv4 server apparatus, the IPv6 terminal can designate the domain nameand communicate with the IPv4 terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a constructional diagram of an IPv4-IPv6 convertingapparatus according to the first embodiment of the invention;

[0024]FIG. 2 is a constructional diagram of an IP address conversiontable;

[0025]FIG. 3 is a constructional diagram of a communication networksystem according to the first embodiment of the invention;

[0026]FIG. 4 is a flowchart in case of starting a communication from anIPv4 terminal to an IPv6 terminal through the IPv4-IPv6 convertingapparatus according to the first embodiment of the invention;

[0027]FIG. 5 is a flowchart which is continuous to FIG. 4;

[0028]FIG. 6 is a flowchart in case of starting a communication from theIPv6 terminal to the IPv4 terminal through the IPv4-IPv6 convertingapparatus according to the first embodiment of the invention;

[0029]FIG. 7 is a flowchart which is continuous to FIG. 6;

[0030]FIG. 8 is a flowchart in another case of starting a communicationfrom the IPv6 terminal to the IPv4 terminal;

[0031]FIG. 9 is a flowchart which is continuous to FIG. 8;

[0032]FIG. 10 is a constructional diagram of another IPv4-IPv6converting apparatus;

[0033]FIG. 11 is a constructional diagram of another network system;

[0034]FIG. 12 is a flowchart in another case of starting a communicationfrom the IPv4 terminal to the IPv6 terminal;

[0035]FIG. 13 is a flowchart which is continuous to FIG. 12;

[0036]FIG. 14 is a flowchart in still another case of starting acommunication from the IPv6 terminal to the IPv4 terminal;

[0037]FIG. 15 is a flowchart which is continuous to FIG. 14;

[0038]FIG. 16 is a constructional diagram of an IPv4-IPv6 convertingapparatus according to the second embodiment of the invention;

[0039]FIG. 17 is a constructional diagram of an IPv4-IPv6 convertingapparatus according to the third embodiment of the invention;

[0040]FIG. 18 is a constructional diagram of an IPv4-IPv6 convertingapparatus according to the fourth embodiment of the invention;

[0041]FIG. 19 is a constructional diagram of a communication networksystem according to the fifth embodiment of the invention;

[0042]FIG. 20 is a flowchart in case of communicating between the IPv6terminals through the IPv4-IPv6 converting apparatus according to thefifth embodiment of the invention; and

[0043]FIG. 21 is a flowchart which is continuous to FIG. 20.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] Embodiments of the present invention will now be describedhereinbelow with reference to the drawings.

[0045] (A) First Embodiment

[0046]FIG. 1 is a constructional diagram of an IPv4-IPv6 convertingapparatus la according to the first embodiment of the invention.

[0047] The IPv4-IPv6 converting apparatus 1 a is located between an IPv4network and an IPv6 network and realizes a communication between an IPv4terminal and an IPv6 terminal by performing an IP header conversion ofan IPv4 packet and an IPv6 packet. The IPv4-IPv6 converting apparatus 1a comprises IP transmitting and receiving means 10, IP header convertingmeans 11, DNS substituting means 13, IPv4 address capturing means 14,and IP address conversion information holding means 15.

[0048] The IP transmitting and receiving means 10 is means fortransmitting and receiving the IPv4 packet and IPv6 packet between theIPv4 network and the IPv6 network and is constructed by, for example,electronic devices such as CPU, RAM, and the like.

[0049] The IP header converting means 11 is means for performing an IPheader conversion of the IPv4 packet and the IPv6 packet and isconstructed by, for example, electronic devices such as CPU, RAM, andthe like.

[0050] The DNS substituting means 13 is means for capturing domaininformation from a DNS server in the IPv4 network or a DNS server in theIPv6 network in accordance with the DNS technique disclosed in RFC1034,RFC1886, or the like and is constructed by, for example, electronicdevices such as CPU, RAM, and the like.

[0051] The IPv4 address capturing means 14 is means for capturing anIPv4 address from a DHCP server in the IPv4 network in accordance withthe DHCP technique disclosed in RFC1541 or the like and is constructedby, for example, electronic devices such as CPU, RAM, and the like.

[0052] The IP address conversion information holding means 15 is meansfor converting the IPv4 address and the IPv6 address and is constructedby, for example, electronic devices such as an RAM and the like. Acorrespondence between the IPv4 address and the IPv6 address is held asan IP address conversion table 151.

[0053]FIG. 2 is a constructional diagram of the IP address conversiontable 151. IPv6 addresses of the IPv6 terminal communicating with theIPv4 terminal and IPv4 addresses which are dynamically allocated incorrespondence to them have been registered in the IP address conversiontable 151.

[0054]FIG. 3 is a diagram of a communication network system 100connecting an IPv4 network 104 and an IPv6 network 106 through theIPv4-IPv6 converting apparatus 1 a.

[0055] In addition to an IPv4 terminal 2, a DNSv4 server 3 for managingdomain information in the IPv4 network 104 and a DHCPv4 server 4 fordynamically allocating the IPv4 addresses to terminals in the IPv4network 104 are connected to the IPv4 network 104.

[0056] In addition to an IPv6 terminal 5, a DNSv6 server 6 for managingdomain information in the IPv6 network is connected to the IPv6 network106.

[0057] It is now assumed that “133. 144. 95. 101” has been allocated asan IPv4 address to the IPv4 terminal 2 and “1 : : 1” has been allocatedas an IPv6 address to the IPv6 terminal 5. It is also assumed that 100IPv4 addresses of “133. 144. 95. 1” to “133. 144. 95. 100”, have beenpooled in the DHCPv4 server 4. It is also assumed that the IPv4-IPv6converting apparatus 1 a has been set as a DNS server into the IPv4terminal 2 and IPv6 terminal 5. Further, it is assumed that the DNSv4server 3 and DNSv6 server 6 have been set as DNS servers and the DHCPv4server 4 has been set as a DHCP server into the IPv4-IPv6 convertingapparatus 1 a.

[0058] In FIG. 3, although the IPv4 network 104 and IPv6 network 106 areperfectly separated, a similar construction is also applied to a casewhere the IPv4 network and IPv6 network physically mixedly exist in thesame network.

[0059]FIGS. 4 and 5 are flowcharts of the operation in case of startinga communication from the IPv4 terminal 2 to the IPv6 terminal 5.

[0060] Although the IPv4 terminal 2 knows a domain name of the IPv6terminal 5, it doesn't know an IP address. As shown in FIG. 4,therefore, after the IPv4 terminal 2 transmitted an inquiry message(hereinafter, called a ‘message A’) of the IP address corresponding tothe domain name of the IPv6 terminal 5 to the IPv4-IPv6 convertingapparatus 1 a via the IPv4 network 104, it waits for a response for apredetermined time.

[0061] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 1 a receives and processes the message A and sendsit to the DNS substituting means 13. The DNS substituting means 13instructs the IP transmitting and receiving means 10 so as to transferthe message A to the DNSv4 server 3, sends the message A thereto, andafter that, waits for a response for a predetermined time. The IPtransmitting and receiving means 10 transfers the message A to the DNSv4server 3 via the IPv4 network.

[0062] The DNSv4 server 3 which received the message A retrieves thedomain information of the IPv6 terminal 5. When it is not registered,however, a response to the message A is not returned.

[0063] The DNS substituting means 13 which didn't receive the responsefrom the DNSv4 server 3 even after the elapse of a predetermined timeinstructs the IP transmitting and receiving means 10 so as to transferthe message A to the DNSv6 server 6, sends the message A thereto, andafter that, waits for a response for a predetermined time. The IPtransmitting and receiving means 10 transfers the message A to the DNSv6server 6 via the IPv6 network.

[0064] The DNSv6 server 6 which received the message A retrieves thedomain information of the IPv6 terminal 5 and returns the registereddomain information of the IPv6 terminal 5 to the IPv4-IPv6 convertingapparatus 1 a as a response message (hereinafter, referred to as a‘message B’) for the message A.

[0065] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 1 a which received the response from the DNSv6server 6 within the predetermined time sends the received message B tothe DNS substituting means 13. The DNS substituting means 13 obtains theIPv6 address (1 : : 1) corresponding to the domain name of the IPv6terminal 5 from the message B and sends it to the IP address conversioninformation holding means 15. The IP address conversion informationholding means 15 retrieves the IP address conversion table 151 by usingthe IPv6 address (1 : : 1) of the IPv6 terminal 5 as a key. If therelevant entry has been registered, the IPv4 address of the entry isreturned to the DNS substituting means 13. On the other hand, when therelevant entry is not registered yet, such a fact is notified to the DNSsubstituting means 13.

[0066] The DNS substituting means 13 which received the IPv4 addressfrom the IP address conversion information holding means 15 forms amessage (hereinafter, referred to as a ‘message E’) to notify of theIPv4 address as an IPv4 address corresponding to the domain name of theIPv6 terminal 5 and sends it to the IP transmitting and receiving means10. The IP transmitting and receiving means 10 transmits the message Eto the IPv4 terminal 2 via the IPv4 network.

[0067] On the other hand, the DNS substituting means 13 which receivedthe notification indicating that the entry is not registered yet fromthe IP address conversion information holding means 15 instructs theIPv4 address capturing means 14 so as to capture the IPv4 address and,after that, waits for a response for a predetermined time. The IPv4address capturing means 14 forms an IPv4 address capturing message(hereinafter, called a ‘message C’), instructs the IP transmitting andreceiving means 10 so as to transfer the message C to the DHCPv4 server4, sends the message C thereto, and after that, waits for a response fora predetermined time. The IP transmitting and receiving means 10transmits the message C to the DHCPv4 server 4 via the IPv4 network.

[0068] The DHCPv4 server 4 which received the message C gives the IPv4address to the IPv6 address (1 : : 1) of the IPv6 terminal 5 (it is nowassumed that “133. 144. 95. 1” has been given) and returns an IPv4address given message (hereinafter, called a ‘message D’).

[0069] The IP transmitting and receiving means 10 which received themessage D sends the message D to the IPv4 address capturing means 14.The IPv4 address capturing means 14 obtains the IPv4 address (133. 144.95. 1) from the message D and notifies the DNS substituting means 13.The DNS substituting means 13 forms the message E to notify of the IPv4address (133. 144. 95. 1) as an IP address corresponding to the domainname of the IPv6 terminal 5 and sends it to the IP transmitting andreceiving means 10. The IP transmitting and receiving means 10 transmitsthe message E to the IPv4 terminal 2 via the IPv4 network. The DNSsubstituting means 13 sends the IPv4 address (133. 144. 95. 1) capturedfrom the DHCPv4 server 4 to the IP address conversion informationholding means 15 in correspondence to the IPv6 address (1 : : 1) of theIPv6 terminal 5. The IP address conversion information holding means 15forms a new entry into the IP address conversion table 151 and registersa correspondence of the IPv4 address (133. 144. 95. 1) and the IPv6address (1 : : 1) of the IPv6 terminal 5. The entry of the IP addressconversion table 151 is deleted in the case where it is not referred fora predetermined time. The IPv4 address (133. 144. 95. 1) is returned tothe DHCPv4 server 4.

[0070] As shown in FIG. 5, the IPv4 terminal 2 transmits the IPv4 packetin which the IPv4 address (133. 144. 95. 1) notified from the IPv4-IPv6converting apparatus 1 a by the message E has been set into an IPv4destination address field of the IPv4 packet and an own IPv4 address(133. 144. 95. 101) has been set into an IPv4 source address field tothe IPv4-IPv6 converting apparatus 1 a through the IPv4 network.

[0071] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 1 a sends the received IPv4 packet to the IP headerconverting means 11. The IP header converting means 11 extracts the IPv4address (133. 144. 95. 101) from the IPv4 source address field of theIPv4 packet, converts the IPv4 address into an ‘IPv4-mapped IPv6address’ (: : FFFF : 133. 144. 95. 101) of 128 bits by adding a fixedpattern of 96 bits mentioned above to the IPv4 address, and sets it intoan IPv6 source address field of the IPv6 packet. The IP headerconverting means 11 extracts the IPv4 address (133. 144. 95. 1) from theIPv4 destination address field of the IPv4 packet and sends it to the IPaddress conversion information holding means 15. The IP addressconversion information holding means 15 obtains the IPv6 address (1 :: 1) corresponding to the IPv4 address (133. 144. 95. 1) with referenceto the IP address conversion table 151 and sends it to the IP headerconverting means 11. The IP header converting means 11 sets the IPv6address (1 : : 1) into an IPv6 destination address field of the IPv6packet. The IPv6 packet formed by performing the IP header conversion asmentioned above is sent to the IP transmitting and receiving means 10.The IP transmitting and receiving means 10 transmits the IPv6 packet tothe IPv6 terminal 5 through the IPv6 network.

[0072] In this manner, the communication from the IPv4 terminal 2 to theIPv6 terminal 5 can be started.

[0073]FIGS. 6 and 7 are flowcharts for the operation in case of startinga communication from the IPv6 terminal 5 to the IPv4 terminal 2.

[0074] Although the IPv6 terminal 5 knows a domain name of the IPv4terminal 2, it doesn't know the IP address. As shown in FIG. 6,therefore, the IPv6 terminal 5 transmits an inquiry message(hereinafter, called a ‘message K’) of an IP address corresponding tothe domain name of the IPv4 terminal 2 to the IPv4-IPv6 convertingapparatus 1 a via the IPv6 network 106 and, after that, waits for aresponse for a predetermined time.

[0075] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 1 a receives and processes the message K and sendsit to the DNS substituting means 13. The DNS substituting means 13instructs the IP transmitting and receiving means 10 so as to transferthe message K to the DNSv6 server 6, sends the message K thereto, andafter that, waits for a response for a predetermined time. The IPtransmitting and receiving means 10 transfers the message K to the DNSv6server 6 via the IPv6 network.

[0076] The DNSv6 server 6 which received the message K retrieves thedomain information of the IPv4 terminal 2. However, when it is notregistered, a response to the message K is not returned.

[0077] The DNS substituting means 13 which didn't receive the responsefrom the DNSv6 server 6 even after the elapse of a predetermined timeinstructs the IP transmitting and receiving means 10 so as to transferthe message K to the DNSv4 server 3, sends the message K thereto, andafter that, waits for a response for a predetermined time. The IPtransmitting and receiving means 10 transfers the message K to the DNSv4server 3 via the IPv4 network.

[0078] The DNSv4 server 3 which received the message K retrieves thedomain information of the IPv4 terminal 2 and returns the registereddomain information of the IPv4 terminal 2 to the IPv4-IPv6 convertingapparatus 1 a as a response message (hereinafter, called a ‘message L’)for the message K.

[0079] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 1 a which received the response from the DNSv4server 3 within the predetermined time sends the received message L tothe DNS substituting means 13. The DNS substituting means 13 obtains theIPv4 address (133. 144. 95. 101) corresponding to the domain name of theIPv4 terminal 2 from the message L.

[0080] Subsequently, the DNS substituting means 13 sends the IPv6address (1 : : 1) of the IPv6 terminal 5 to the IP address conversioninformation holding means 15. The IP address conversion informationholding means 15 retrieves the IP address conversion table 151 by usingthe IPv6 address (1 : : 1) of the IPv6 terminal 5 as a key and notifiesthe DNS substituting means 13 of information about whether the relevantentry has been registered or not.

[0081] The DNS substituting means 13 which received a notificationindicating that the relevant entry is not registered yet in the IPv6address (1 : : 1) of the IPv6 terminal 5 instructs the IPv4 addresscapturing means 14 so as to capture the IPv4 address and, after that,waits for a response for a predetermined time. The IPv4address capturingmeans 14 forms an IPv4 address capture message (hereinafter, called a‘message M’), instructs the IP transmitting and receiving means 10 so asto transfer the message M to the DHCPv4 server 4, sends the message Mthereto, and after that, waits for a response for a predetermined time.The IP transmitting and receiving means 10 transmits the message M tothe DHCPv4 server 4 via the IPv4 network.

[0082] The DHCPv4 server 4 which received the message M gives the IPv4address to the IPv6 address (1 : : 1) of the IPv6 terminal 5 (it is nowassumed that “133. 144. 95. 1” is given) and returns an IPv4 addressgiven message (hereinafter, called a ‘message N’).

[0083] The IP transmitting and receiving means 10 which received themessage N sends the message N to the IPv4 address capturing means 14.The IPv4 address capturing means 14 obtains the IPv4 address (133. 144.95. 1) from the message N and notifies the DNS substituting means 13.The DNS substituting means 13 sends the IPv4 address (133. 144. 95. 1)captured from the DHCPv4 server 4 to the IP address conversioninformation holding means 15 in correspondence to the IPv6 address (1 :: 1) of the IPv6 terminal 5. The IP address conversion informationholding means 15 forms a new entry into the IP address conversion table151 and registers a correspondence of the IPv4 address (133. 144. 95. 1)and the IPv6 address (1 : : 1) of the IPv6 terminal 5. The entry in theIP address conversion table 151 is deleted when it is not referred tofor a predetermined time and the IPv4 address (133. 144. 95. 1) isreturned to the DHCPv4 server 4.

[0084] When a notification indicating that the relevant entry has beenregistered in the IPv6 address (1 : : 1) of the IPv6 terminal 5 isreceived or when the registration of the relevant entry into the IPv6address (1 : : 1) of the IPv6 terminal 5 is completed, the DNSsubstituting means 13 forms a message (hereinafter, called a ‘messageP’) to notify of an ‘IPv4-mapped IPv6 address’ (: : FFFF : 133. 144. 95.101) in which the IPv4 address (133. 144. 95. 101) of the IPv4 terminal2 has been set as an IP address corresponding to the domain name of theIPv4 terminal 2. The DNS substituting means 13 instructs the IPtransmitting and receiving means 10 so as to transfer the message P tothe IPv6 terminal 5 and sends the message P thereto. The IP transmittingand receiving means 10 transmits the message P to the IPv6 terminal 5via the IPv6 network.

[0085] As shown in FIG. 7, the IPv6 terminal 5 transmits the IPv6 packetin which the IPv6 address (: : FFFF : 133. 144. 95. 101) notified fromthe IPv4-IPv6 converting apparatus 1 a has been set into the IPv6destination address field of the IPv6 packet and the own IPv6 address (1: : 1) has been set into the IPv6 source address field to the IPv4-IPv6converting apparatus 1 a through the IPv6 network.

[0086] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 1 a sends the received IPv6 packet to the IP headerconverting means 11. The IP header converting means 11 extracts the IPv6address ( : : FFFF : 133. 144. 95. 101) from the IPv6 destinationaddress field of the IPv6 packet, converts the IPv6 address into theIPv4 address (133. 144. 95. 101) of 32 bits by deleting the fixedpattern of 96 bits mentioned above, and sets the IPv4 address into theIPv4 destination address field of the IPv4 packet. The IP headerconverting means 11 extracts the IPv6 address (1 : : 1) from the IPv6source address field of the IPv6 packet and sends it to the IP addressconversion information holding means 15. The IP address conversioninformation holding means 15 obtains the IPv4 address (133. 144. 95. 1)corresponding to the IPv6 address (1 : : 1) with reference to the IPaddress conversion table 151 and sends it to the IP header convertingmeans 11. The IP header converting means 11 sets the IPv4 address (133.144. 95. 1) into the IPv4 source address field of the IPv4 packet. TheIPv4 packet formed by performing the IP header conversion as mentionedabove is sent to the IP transmitting and receiving means 10. The IPtransmitting and receiving means 10 transmits the IPv4 packet to theIPv4 terminal 2 through the IPv4 network.

[0087] In this manner, the communication from the IPv6 terminal 5 to theIPv4 terminal 2 can be started.

[0088] Although the case where the IPv4 domain information had beenregistered in the DNSv4 server 3 has been described above, the IPv4domain information can be also registered in the DNSv6 server 6. In thiscase, the IPv4-IPv6 converting apparatus 1 a captures the IPv4 domaininformation by communicating with the DNSv6 server 6 by using the IPv6protocol. Similarly, the IPv6 domain information can be also registeredin the DNSv4 server 3. In this case, the IPv4-IPv6 converting apparatus1 a captures the IPv6 domain information by communicating with the DNSv4server 3 by using the IPv4protocol.

[0089] Explanation has been made with respect to the case where theaddress conversion is performed solely in the address field by adding ordeleting the fixed pattern of 96 bits mentioned above or by referring tothe IP address conversion table 151. As for the address on the terminalside which started the communication, however, a combination of theaddress on the terminal side which started the communication and theport number of a TCP (Transmission Control Protocol)/UDP (User DatagramProtocol) can be also converted to a combination of an address of theIPv4-IPv6 converting apparatus and a port number of the TCP/UDP by usinga method called an IP masquerade.

[0090]FIGS. 8 and 9 are flowcharts for operation in another case ofstarting a communication from the IPv6 terminal 5 to the IPv4 terminal2. This case has a feature in that temporary allocation of IPv4 addressto the IPv6 terminal is performed at the time of actually starting adata communication.

[0091] Although the IPv6 terminal 5 knows a domain name of the IPv4terminal 2, it doesn't know the IP address. As shown in FIG. 8,therefore, the IPv6 terminal 5 transmits an inquiry message(hereinafter, called a ‘message K’) of an IP address corresponding tothe domain name of the IPv4 terminal 2 to the IPv4-IPv6 convertingapparatus 1 a via the IPv6 network 106 and, after that, waits for aresponse for a predetermined time.

[0092] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 1 a receives and processes the message K and sendsit to the DNS substituting means 13. The DNS substituting means 13instructs the IP transmitting and receiving means 10 so as to transferthe message K to the DNSv6 server 6, sends the message K thereto, andafter that, waits for a response for a predetermined time. The IPtransmitting and receiving means 10 transfers the message K to the DNSv6server 6 via the IPv6 network.

[0093] The DNSv6 server 6 which received the message K retrieves thedomain information of the IPv4 terminal 2. However, when it is notregistered, a response to the message K is not returned.

[0094] The DNS substituting means 13 which didn't receive the responsefrom the DNSv6 server 6 even after the elapse of a predetermined timeinstructs the IP transmitting and receiving means 10 so as to transferthe message K to the DNSv4 server 3, sends the message K thereto, andafter that, waits for a response for a predetermined time. The IPtransmitting and receiving means 10 transfers the message K to the DNSv4server 3 via the IPv4 network.

[0095] The DNSv4 server 3 which received the message K retrieves thedomain information of the Ipv4 terminal 2 and returns the registereddomain information of the IPv4 terminal 2 to the IPv4-IPv6 convertingapparatus 1 a as a response message (hereinafter, called a ‘message L’)for the message K.

[0096] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 1 a which received the response from the DNSv4server 3 within the predetermined time sends the received message L tothe DNS substituting means 13. The DNS substituting means 13 obtains theIPv4 address (133. 144. 95. 101) corresponding to the domain name of theIPv4 terminal 2 from the message L.

[0097] The DNS substituting means 13 forms a message (hereinafter,called a ‘message P’) to notify of an ‘IPv4-mapped IPv6 address’ (: :FFFF : 133. 144. 95. 101) in which the IPv4 address (133. 144. 95. 101)of the IPv4 terminal 2 has been set as an IP address corresponding tothe domain name of the IPv4 terminal 2. The DNS substituting means 13instructs the IP transmitting and receiving means 10 so as to transferthe message P to the IPv6 terminal 5 and sends the message P thereto.The IP transmitting and receiving means 10 transmits the message P tothe IPv6 terminal 5 via the IPv6 network.

[0098] As shown in FIG. 9, the IPv6 terminal 5 transmits the IPv6 packetin which the IPv6 address (: : FFFF : 133. 144. 95. 101) notified fromthe IPv4-IPv6 converting apparatus 1 a has been set-into the IPv6destination address field of the IPv6 packet and the own IPv6 address (1: : 1) has been set into the IPv6 source address field to the IPv4-Ipv6converting apparatus 1 a through the IPv6 network.

[0099] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 1 a sends the received IPv6 packet to the IP headerconverting means 11. The IP header converting means 11 extracts the IPv6address (: : FFFF : 133. 144. 95. 101) from the Ipv6 destination addressfield of the IPv6 packet, converts the IPv6 address into the IPv4address (133. 144. 95. 101) of 32 bits by deleting the fixed pattern of96 bits mentioned above, and sets the IPv4 address into the IPv4destination address field of the IPv4 packet. The IP header convertingmeans 11 extracts the IPv6 address (1 : : 1) from the IPv6 sourceaddress field of the IPv6 packet and sends it to the IP addressconversion information holding means 15.

[0100] The IP address conversion information holding means 15 retrievesthe IP address conversion table 151 by using the IPv6 address (1 : : 1)of the IPv6 terminal 5 as a key and notifies the IP header convertingmeans 11 of a result indicating whether the relevant entry has beenregistered or not.

[0101] The IP header converting means 11 which received the notificationthat the relevant entry is not registered yet in the IPv6 address (1 :: 1) of the IPv6 terminal 5 instructs the IPv4 address capturing means14 so as to capture the IPv4 address and, after that, waits for a replyfor a predetermined time. The IPv4 address capturing means 14 forms anIPv4 address capturing message (hereinafter, referred to as a “messageM”), instructs the IP transmitting and receiving means 10 s as totransfer the message M to the DHCPv4 server 4, supplies the message Mthereto, and after that, waits for a reply for a predetermined time. TheIP transmitting and receiving means 10 transmits the message M to theDHCPv4 server 4 via the IPv4 network.

[0102] The DHCPv4 server 4 which received the message M gives the IPv4address to the IPv6 address (1 : : 1) of the IPv6 terminals 5 (it is nowassumed that 133. 144. 95. 1 was given) and returns an IPv4 addressgiven message (hereinafter, referred to as a “message N”).

[0103] The IP transmitting and receiving means 10 which received themessage N transmits the message N to the IPv4 address capturing means14. The IPv4 address capturing means 14 obtains the IPv4 address (133.144. 95. 1) from the message N and notifies the IP header convertingmeans 11. The IP header converting means 11 transmits the IPv4 address(133. 144. 95. 1) captured from the DHCPv4 server 4 to the IP addressconversion information holding means 15 so as to correspond to the IPv6address (1 : : 1) of the IPv6 terminal 5. The IP address conversioninformation holding means 15 forms a new entry into the IP addressconversion table 151 and registers the correspondence between the IPv4address (133. 144. 95. 1) and the IPv6 address (1 : : 1) of the IPv6terminal 5. The entry of the IP address conversion table 151 is deletedwhen it is not referred to for a predetermined time. The IPv4 address(133. 144. 95. 1) is returned to the DHCPv4 server 4.

[0104] When a notification that the corresponding entry has beenregistered in the IPv6 address (1 : : 1) of the IPv6 terminal 5 isreceived or when the registration of the corresponding entry into theIPv6 address (1 : : 1) of the IPv6 terminal 5 is completed, the IPheader converting means 11 sets the IPv4 address (133. 144. 95. 1) intothe IPv4 source address field of the IPv4 packet. The IPv4 packet formedby performing the IP header conversion as mentioned above is sent to theIP transmitting and receiving means 10. The IP transmitting andreceiving means 10 transmits the IPv4 packet to the IPv4 terminal 2through the IPv4 network.

[0105] In this manner, the communication from the IPv6 terminal 5 to theIPv4 terminal 2 can be started.

[0106]FIGS. 12 and 13 are flowcharts for operation in another case ofstarting a communication from the IPv4 terminal 2 to the IPv6 terminal5. This case has a feature in that “IPv4-mapped IPv6 address” is notused.

[0107] Although the IPv4 terminal 2 knows a domain name of the IPv6terminal 5, it doesn't know an IP address. As shown in FIG. 12,therefore, after the IPv4 terminal 2 transmitted an inquiry message(hereinafter, called a ‘message A’) of the IP address corresponding tothe domain name of the IPv6 terminal 5 to the IPv4-IPv6 convertingapparatus 1 a via the IPv4 network 104, it waits for a response for apredetermined time.

[0108] The IP transmitting and receiving means 10 of the IPv4-Ipv6converting apparatus 1 a receives and processes the message A and sendsit to the DNS substituting means 13. The DNS substituting means 13instructs the IP transmitting and receiving means 10 so as to transferthe message A to the DNSv4 server 3, sends the message A thereto, andafter that, waits for a response for a predetermined time. The IPtransmitting and receiving means 10 transfers the message A to the DNSv4server 3 via the IPv4 network.

[0109] The DNSv4 server 3 which received the message A retrieves thedomain information of the IPv6 terminal 5. When it is not registered,however, a response to the message A is not returned.

[0110] The DNS substituting means 13 which didn't receive the responsefrom the DNSv4 server 3 even after the elapse of a predetermined timeinstructs the IP transmitting and receiving means 10 so as to transferthe message A to the DNSv6 server 6, sends the message A thereto, andafter that, waits for a response for a predetermined time. The IPtransmitting and receiving means 10 transfers the message A to the DNSv6server 6 via the IPv6 network.

[0111] The DNSv6 server 6 which received the message A retrieves thedomain information of the IPv6 terminal 5 and returns the registereddomain information of the IPv6 terminal 5 to the IPv4-IPv6 convertingapparatus 1 a as a response message (hereinafter, referred to as a‘message B’) for the message A.

[0112] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 1 a which received the response from the DNSv6server 6 within the predetermined time sends the received message B tothe DNS substituting means 13. The DNS substituting means 13 obtains theIPv6 address (1 : : 1) corresponding to the domain name of the IPv6terminal 5 from the message B and sends it to the IP address conversioninformation holding means 15. The IP address conversion informationholding means 15 retrieves the IP address conversion table 151 by usingthe IPv6 address (1 : : 1) of the IPv6 terminal 5 as a key. If therelevant entry has been registered, the IPv4 address of the entry isreturned to the DNS substituting means 13. On the other hand, when therelevant entry is not registered yet, such a fact is notified to the DNSsubstituting means 13.

[0113] The DNS substituting means 13 which received the IPv4 addressfrom the Ip address conversion information holding means 15 forms amessage (hereinafter, referred to as a ‘message E’) to notify of theIPv4 address as an IPv4 address corresponding to the domain name of theIPv6 terminal 5 and sends it to the IP transmitting and receiving means10. The IP transmitting and receiving means 10 transmits the message Eto the IPv4 terminal 2 via the IPv4 network.

[0114] On the other hand, the DNS substituting means 13 which receivedthe notification indicating that the entry is not registered yet fromthe IP address conversion information holding means 15 instructs theIPv4 address capturing means 14 so as to capture the IPv4 address and,after that, waits for a response for a predetermined time. The IPv4address capturing means 14 forms an IPv4 address capturing message(hereinafter, called a ‘message C’) instructs the IP transmitting andreceiving means 10 so as to transfer the message C to the DHCPv4 server4, sends the message C thereto, and after that, waits for a response fora predetermined time. The IP transmitting and receiving means 10transmits the message C to the DHCPv4 server 4 via the IPv4 network.

[0115] The DHCPv4 serve 4 which received the message C gives the IPv4address to the IPv6 address (1 : : 1) of the IPv6 terminal 5 (it is nowassumed that “133. 144. 95. 1” has been given) and returns an IPv4address given message (hereinafter, called a ‘message D’).

[0116] The IP transmitting and receiving means 10 which received themessage D sends the message D to the IPv4 address capturing means 14.The IPv4 address capturing means 14 obtains the IPv4 address (133. 144.95. 1) from the message D and notifies the DNS substituting means 13.The DNS substituting means 13 forms the message E to notify of the IPv4address (133. 144. 95. 1) as an IP address corresponding to the domainname of the IPv6 terminal 5 and sends it to the IP transmitting andreceiving means 10. The IP transmitting and receiving means 10 transmitsthe message E to the IPv4 terminal 2 via the IPv4 network. The DNSsubstituting means 13 sends the IPv4 address (133. 144. 95. 1) capturedfrom the DHCPv4 server 4 to the IP address conversion informationholding means 15 in correspondence to the IPv6 address (1 : : 1) of theIPv6 terminal 5. The IP address conversion information holding means 15forms a new entry into the IP address conversion table 151 and registersa correspondence of the IPv4 address (133. 144. 95. 1) and the IPv6address (1 : : 1) of the IPv6 terminal 5. The entry of the IP addressconversion table 151 is deleted in the case where it is not referred fora predetermined time. The IPv4 address (133. 144. 95. 1) is returned tothe DHCPv4 server 4.

[0117] As shown in FIG. 13, the IPv4 terminal 2 transmits the IPv4packet in which the IPv4 address (133. 144. 95. 1) notified from theIPv4-IPv6 converting apparatus 1 a by the message E has been set into anIPv4 destination address field of the IPv4 packet and an own IPv4address (133. 144. 95. 101) has been set into an IPv4 source addressfield to the IPv4-IPv6 converting apparatus 1 a through the IPv4network.

[0118] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 1 a sends the received IPv4 packet to the IP headerconverting means 11. The IP header converting means 11 extracts the IPv4address (133. 144. 95. 101) from the IPv4 source address field of theIPv4 packet, and sends it to the IP address conversion informationholding means 15. The IP address conversion information holding means 15retrieves the IP address conversion table 151 by using the IPv4 address(133. 144. 95. 101) of the IPv4 terminal 2 as a key. When the relevantentry has been registered, the IPv6 address of the entry is returned tothe IP header converting means 11. On the other hand, if the relevantentry is not registered yet, such a fact is notified to the IP headerconverting means 11.

[0119] The IP header converting means 11 which received the notificationthat the entry is not registered from the IP address conversioninformation holding means 15 instructs the IPv6 address capturing means20 so as to capture the IPv6 address and, after that, waits for a replyfor a predetermined time. The IPv6 address capturing means 20 forms anIPv6 address capturing message (hereinafter, referred to as a “messageC6”), instructs the IP transmitting and receiving means 10 so as totransfer the message C6 to the DHCPv6 server 7 (FIG. 11), supplies themessage C6 thereto, and after that, waits a replay for a predeterminedtime. The IP transmitting and receiving means 10 transmits the messageC6 to the DHCPv6 server 7 via the IPv6 network.

[0120] The DHCPv6 server 7 which received the message C6 gives the IPv6address to the IPv4 address (133. 144. 95. 101) of the IPv4 terminal 2(it is now assumed that 1 : : 101 was given) and returns an IPv6 addressgiven message (hereinafter, referred to as a “message D6”).

[0121] The IP transmitting and receiving means 10 which received themessage D6 supplies the message D6 to the IPv6 address capturing means20 (FIG. 10). The IPv6 address capturing means 20 obtains an IPv6address (1 : : 101) from the message D6 and notifies the IP headerconverting means 11.

[0122] The IP header converting means 11 transmits the IPv6 address (1 :: 101) captured from the DHCPv6 server 7 to the IP address conversioninformation holding means 15 so as to correspond to the IPv4 address(133. 144. 95. 101) of the IPv4 terminal 2. The IP address conversioninformation holding means 15 forms a new entry into the IP addressconversion table 151 and registers a correspondence between the IPv6address (1 : : 101) and the IPv4 address (133. 144. 95. 101) of the IPv4terminal 2. The entry in the IP address conversion table 151 is deletedwhen it is not referred to for a predetermined time. The IPv6 address (1: : 101) is returned to the DHCPv6 server 7.

[0123] When a notification that the relevant entry has been registeredin the IPv4 address (133. 144. 95. 101) of the IPv4 terminal 2 isreceived or when the registration of the relevant entry into the IPv4address (133. 144. 95. 101) of the IPv4 terminal 2 is completed, theIPv6 address (1 : : 101) of the relevant entry is set into an IPv6transmission address field of the IPv6 packet. The IP header convertingmeans 11 extracts the IPv4 address (133. 144. 95. 1) from the IPv4destination address field of the IPv4 packet and sends it to the IPaddress conversion information holding means 15. The IP addressconversion information holding means 15 obtains the IPv6 address (1 :: 1) corresponding to the IPv4 address (133. 144. 95. 1) with referenceto the IP address conversion table 151 and sends it to the IP headerconverting means 11. The IP header converting means 11 sets the IPv6address (1 : : 1) into an IPv6 destination address field of the IPv6packet. The IPv6 packet formed by performing the IP header conversion asmentioned above is sent to the IP transmitting and receiving means 10.The IP transmitting and receiving means 10 transmits the IPv6 packet tothe IPv6 terminal 5 through the IPv6 network.

[0124] In this manner, the communication from the IPv4 terminal 2 to heIPv6 terminal 5 can be started.

[0125]FIGS. 14 and 15 are flowcharts for operation in another case ofstarting a communication from the IPv6 terminal 5 to the IPv4 terminal2. This case has the feature in that the “IPv4-mapped IPv6 address” isnot used.

[0126] Although the IPv6 terminal 5 knows a domain name of the IPv4terminal 2, it doesn't know the IP address. As shown in FIG. 14,therefore, the IPv6 terminal 5 transmits an inquiry message(hereinafter, called a ‘message K’) of an IP address corresponding tothe domain name of the IPv4 terminal 2 to the IPv4-IPv6 convertingapparatus 1 a via the IPv6 network 106 and, after that, waits for aresponse for a predetermined time.

[0127] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 1 a receives and processes the message K and sendsit to the DNS substituting means 13. The DNS substituting means 13instructs the IP transmitting and receiving means 10 so as to transferthe message K to the DNSv6 server 6, sends the message K thereto, andafter that, waits for a response for a predetermined time. The IPtransmitting and receiving means 10 transfers the message K to the DNSv6server 6 via the IPv6 network.

[0128] The DNSv6 server 6 which received the message K retrieves thedomain information of the IPv4 terminal 2. However, when it is notregistered, a response to the message K is not returned.

[0129] The DNS substituting means 13 which didn't receive the responsefrom the DNSv6 server 6 even after the elapse of a predetermined timeinstructs the IP transmitting and receiving means 10 so as to transferthe message K to the DNSv4 server 3, sends the message K thereto, andafter that, waits for a response for a predetermined time. The IPtransmitting and receiving means 10 transfers the message K to the DNSv4server 3 via the IPv4 network.

[0130] The DNSv4 server 3 which received the message K retrieves thedomain information of the IPv4 terminal 2 and returns the registereddomain information of the IPv4 terminal 2 to the IPv4-IPv6 convertingapparatus 1 a as a response message (hereinafter, called a ‘message L’)for the message K.

[0131] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 1 a which received the response from the DNSv4server 3 within the predetermined time sends the received message L tothe DNS substituting means 13. The DNS substituting means 13 obtains theIPv4 address (133. 144. 95. 101) corresponding to the domain name of theIPv4 terminal 2 from the message L.

[0132] Subsequently, the DNS substituting means 13 sends the IPv4address (133. 144. 95. 101) of the IPv4 terminal 2 to the IP addressconversion information holding means 15. The IP address conversioninformation holding means 15 retrieves the IP address conversion table151 by using the IPv4 address (133. 144. 95. 101) of the IPv4 terminal 2as a key and notifies the DNS substituting means 13 of information aboutwhether the relevant entry has been registered or not.

[0133] The DNS substituting means 13 which received a notificationindicating that the relevant entry is not registered yet in the IPv4address (133. 144. 95. 101) of the IPv4 terminal 2 instructs the IPv6address capturing means 20 so as to capture the IPv6 address and, afterthat, waits for a response for a predetermined time. The IPv6 addresscapturing means 20 forms an IPv6 address capture message (hereinafter,called a ‘message M6’), instructs the IP transmitting and receivingmeans 10 so as to transfer the message M to the DHCPv6 server 7, sendsthe message M thereto, and after that, waits for a response for apredetermined time. The IP transmitting and receiving means 10 transmitsthe message M6 to the DHCPv6 server 7 via the IPv6 network.

[0134] The DHCPv6 server 7 which received the message M6 gives the IPv6address to the IPv4 address (133. 144. 95. 101) of the IPv4 terminal 2(it is now assumed that “1 : : 101” is given) and returns an IPv6address given message (hereinafter, called a ‘message N6’).

[0135] The IP transmitting and receiving means 10 which received themessage N6 sends the message N6 to the IPv6 address capturing means 20.The IPv6 address capturing means 20 obtains the IPv6 address (1 : : 101)from the message N6 and notifies the DNS substituting means 13. The DNSsubstituting means 13 sends the IPv6 address (1 : 101) captured from theDHCPv6 server 7 to the IP address conversion information holding means15 in correspondence to the IPv4 address (133. 144. 95. 101) of the IPv4terminal 2. The IP address conversion information holding means 15 formsa new entry into the IP address conversion table 151 and registers acorrespondence of the IPv6 address (1 : : 101) and the IPv4 address(133. 144. 95. 101) of the IPv4 terminal 2. The entry in the IP addressconversion table 151 is deleted when it is not referred to for apredetermined time and the IPv6 address (1 : : 101) is returned to theDHCPv6 server 7.

[0136] When a notification indicating that the relevant entry has beenregistered in the IPv4 address (133. 144. 95. 101) of the IPv4 terminal2 is received or when the registration of the relevant entry into theIPv4 address (133. 144. 95. 101) of the IPv4 terminal 2 is completed,the DNS substituting means 13 forms a message (hereinafter, called a‘message P’) to notify of the IPv6 address (1 : : 101) corresponding tothe IPv4 address (133. 144. 95. 101) of the IPv4 terminal 2 as an IPaddress corresponding to the domain name of the IPv4 terminal 2. The DNSsubstituting means 13 instructs the IP transmitting and receiving means10 so as to transfer the message P to the IPv6 terminal 5 and sends themessage P thereto. The IP transmitting and receiving means 10 transmitsthe message P to the IPv6 terminal 5 via the IPv6 network.

[0137] As shown in FIG. 15, the IPv6 terminal 5 transmits the IPv6packet in which the IPv6 address (1 : : 101) notified from the IPv4-IPv6converting apparatus 1 a has been set into the IPv6 destination addressfield of the IPv6 packet and the own IPv6 address (1 : : 1) has been setinto the IPv6 source address field to the IPv4-IPv6 converting apparatus1 a through the IPv6 network.

[0138] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 1 a sends the received IPv6 packet to the IP headerconverting means 11. The IP header converting means 11 extracts the IPv6address (1 : : 101) from the IPv6 destination address field of the Ipv6packet, and supplies it to the IP address conversion information holdingmeans 15. The IP address conversion information holding means 15 obtainsthe IPv4 address (133. 144. 95. 101) corresponding to the IPv6 address(1 : : 101) with reference to the IP address conversion table 151 andtransmits to the IP header converting means 11. The IP header convertingmeans 11 sets the IPv4 address (133. 144. 95. 101) into the IPv4destination address field of the IPv4 packet. The IP header convertingmeans 11 extracts the IPv6 address (1 : : 1) from the IPv6 sourceaddress field of the IPv6 packet and sends it to the IP addressconversion information holding means 15.

[0139] The IP address conversion information holding means 15 retrievesthe IP address conversion table 151 by using the IPv6 address (1 : : 1)of the IPv6 terminal 5 as a key and notifies the IP header convertingmeans 11 of a result indicating whether the relevant entry has beenregistered or not.

[0140] The IP header converting means 11 which received the notificationthat the relevant entry is not registered yet in the IPv6 address (1 :: 1) of the IPv6 terminal 5 instructs the IPv4 address capturing means14 so as to capture the Ipv4 address and, after that, waits for a replyfor a predetermined time. The IPv4 address capturing means 14 forms anIPv4 address capturing message (hereinafter, referred to as a “messageM”), instructs the IP transmitting and receiving means 10 so as totransfer the message M to the DHCPv4 server 4, supplies the message Mthereto, and after that, waits for a reply for a predetermined time. TheIP transmitting and receiving means 10 transmits the message M to theDHCPv4 server 4 via the IPv4 network.

[0141] The DHCPv4 server 4 which received the message M gives the IPv4address to the IPv6 address (1 : : 1) of the IPv6 terminal 5 (it is nowassumed that 133. 144. 95. 1 was given) and returns an IPv4 addressgiven message (hereinafter, referred to as a “message N”).

[0142] The IP transmitting and receiving means 10 which received themessage N transmits the message N to the IPv4 address capturing means14. The IPv4 address capturing means 14 obtains the IPv4 address (133.144. 95. 1) from the message N and notifies the IP header convertingmeans 11. The IP header converting means 11 transmits the IPv4 address(133. 144. 95. 1) captured from the DHCPv4 server 4 to the IP addressconversion information holding means 15 so as to correspond to the IPv6address (1 : : 1) of the IPv6 terminal 5. The IP address conversioninformation holding means 15 forms a new entry into the IP addressconversion table 151 and registers the correspondence between the IPv4address (133. 144. 95. 1) and the IPv6 address (1 : : 1) of the IPv6terminal 5. The entry of the IP address conversion table 151 is deletedwhen it is not referred to for a predetermined time. The IPv4 address(133. 144. 95. 1) is returned to the DHCPv4 server 4.

[0143] When a notification that the corresponding entry has beenregistered in the IPv6 address (1 : : 1) of the IPv6 terminal 5 isreceived or when the registration of the corresponding entry into theIPv6 address (1 : : 1) of the IPv6 terminal 5 is completed, the IPheader converting means 11 sets the IPv4 address (133. 144. 95. 1) intothe IPv4 source address field of the IPv4 packet. The IPv4 packet formedby performing the IP header conversion as mentioned above is sent to theIP transmitting and receiving means 10. The IP transmitting andreceiving means 10 transmits the IPv4 packet to the IPv4 terminal 2through the Ipv4 network.

[0144] In this manner, the communication from the IPv6 terminal 5 to theIPv4 terminal 2 can be started.

[0145] (B) Second Embodiment

[0146]FIG. 16 is a constructional diagram of an IPv4-IPv6 convertingapparatus 1 b according to the second embodiment of the invention.

[0147] The IPv4-IPv6 converting apparatus 1 b has a constructionobtained by adding IPv4 address pool means 16 to the IPv4-IPv6converting apparatus 1 a of FIG. 1.

[0148] At the time of an activation or the like of the IPv4-IPv6converting apparatus 1 b, the IPv4 address capturing means 14 captures aplurality of IPv4 addresses in a lump from the DHCPv4 server 4 and poolsthem into the IPv4 address pool means 16. At the start of thecommunication between the IPv4 terminal 2 and IPv6 terminal 5, the IPv4address capturing means 14 obtains the IPv4 addresses from the IPv4address pool means 16.

[0149] By using the IPv4-IPv6 converting apparatus 1 b, a load of theIPv4 network 104 can be reduced more than the case of capturing the IPv4addresses one by one from the DHCPv4 server 4.

[0150] (C) Third Embodiment

[0151]FIG. 17 is a constructional diagram of an IPv4-IPv6 convertingapparatus 1 c according to the third embodiment of the invention.

[0152] The IPv4-IPv6 converting apparatus 1 c has a constructionobtained by providing DHCPv4 server means 17 in place of the IPv4address capturing means 14 of the IPv4-IPv6 converting apparatus 1 a ofFIG. 1.

[0153] By using the IPv4-IPv6 converting apparatus 1 c, there is no needto individually provide the DHCPv4 server 4.

[0154] (D) Fourth Embodiment

[0155]FIG. 18 is a constructional diagram of an IPv4-IPv6 convertingapparatus id according to the fourth embodiment of the invention.

[0156] The IPv4-IPv6 converting apparatus 1 d has a constructionobtained by adding DNSv4 server means 18 and DNSv6 server means 19 tothe IPv4-IPv6 converting apparatus 1 a of FIG. 1.

[0157] By using the IPv4-IPv6 converting apparatus id, there is no needto individually provide the DNSv4 server 3 and DNSv6 server 6.

[0158] (E) Fifth Embodiment

[0159] The fifth embodiment will now be described by using FIGS. 19, 20,and 21.

[0160]FIG. 19 is a diagram of a communication network system 108 inwhich the IPv6 network 106 and an IPv6 network 107 are connected throughthe IPv4 network 104. The communication network system 108 has aconstruction obtained by adding the IPv6 network 107 to thecommunication network system 100 of FIG. 3. The IPv6 network 107 isconnected to the IPv4 network 104 by an IPv4-IPv6 converting apparatus111.

[0161] In addition to an IPv6 terminal 115, a DNSv6 server 116 formanaging domain information in the IPv6 network 107 is connected to theIPv6 network 107.

[0162] It is now assumed that (2 : : 1) has been allocated as an IPv6address to the IPv6 terminal 115. In addition to 100 IPv4 addresses of“133. 144. 95. 1” to “133. 144. 95. 100”, 100 IPv4 addresses of “133.144. 96. 1” to “133. 144. 96. 100” have been pooled in the DHCPv4 server4. It is also assumed that the IPv4-IPv6 converting apparatus 111 hasbeen set as a DNS server in the IPv6 terminal 115. It is assumed thatthe DNSv6 server 116 has been set as a DNS server and the DHCPv4 server4 has been set as a DHCP server into the IPv4-IPv6 converting apparatus111. It is also assumed that the IPv4-IPv6 converting apparatus 111 hasbeen set as a DNS server of an upper domain into an IPv4-IPv6 convertingapparatus 1.

[0163]FIGS. 20 and 21 are flowcharts for the operation in case ofstarting a communication from the IPv6 terminal 5 to the IPv6 terminal115.

[0164] Although the IPv6 terminal 5 knows a domain name of the IPv6terminal 115, it doesn't know an IP address. Therefore, as shown in FIG.20, the IPv6 terminal 5 transmits an inquiry message (hereinafter,called a ‘message Q’) of the IP address corresponding to the domain nameof the IPv6 terminal 115 to the IPv4-IPv6 converting apparatus 1 via theIPv6 network 106 and, after that, waits for a response for apredetermined time.

[0165] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 1 receives and processes the message Q and sends itto the DNS substituting means 13. The DNS substituting means 13instructs the IP transmitting and receiving means 10 so as to transferthe message Q to the IPv4-IPv6 converting apparatus 111, sends themessage Q thereto, and after that, waits for a response for apredetermined time. The IP transmitting and receiving means 10 transfersthe message Q to the IPv4-IPv6 converting apparatus 111 via the IPv4network.

[0166] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 111 receives and processes the message Q and sendsit to the DNS substituting means 13. The DNS substituting means 13instructs the IP transmitting and receiving means 10 so as to transferthe message Q to the DNSv6 server 116, sends the message Q thereto, andafter that, waits for a response for a predetermined time. The IPtransmitting and receiving means 10 transfers the message Q to the DNSv6server 116 via the IPv6 network.

[0167] The DNSv6 server 116 which received the message Q retrievesdomain information of the IPv6 terminal 115 and returns the registereddomain information of the IPv6 terminal 115 to the IPv4-IPv6 convertingapparatus 111 as a response message (hereinafter, called a ‘message R’)for the message Q.

[0168] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 111 which received the response from the DNSv6server 116 within a predetermined time sends the received message R tothe DNS substituting means 13. The DNS substituting means 13 obtains anIPv6 address (2 : : 1) corresponding to the domain name of the IPv6terminal 115 from the message R and sends it to the IP addressconversion information holding means 15. The IP address conversioninformation holding means 15 retrieves the IP address conversion table151 by using the IPv6 address (2 : : 1) of the IPv6 terminal 115 as akey. If the relevant entry has been registered, the holding means 15returns the IPv4 address of the entry to the DNS substituting means 13.On the other hand, if the relevant entry is not registered yet, such afact is notified to the DNS substituting means 13.

[0169] The DNS substituting means 13 which received the IPv4 addressfrom the IP address conversion information holding means 15 forms amessage (hereinafter, called a ‘message U’) to notify of the IPv4address as an IPv4 address corresponding to the domain name of the IPv6terminal 115 and sends the message U to the IP transmitting andreceiving means 10. The IP transmitting and receiving means 10 transmitsthe message U to the IPv4-IPv6 converting apparatus 1 via the IPv4network.

[0170] The DNS substituting means 13 which received the notificationthat the entry is not registered yet from the IP address conversioninformation holding means 15 instructs the IPv4 address capturing means14 so as to capture the IPv4 address and, after that, waits for aresponse for a predetermined time. The IPv4 address capturing means 14forms an IPv4 address capturing message (hereinafter, called a ‘messageS’), instructs the IP transmitting and receiving means 10 so as totransfer the message S to the DHCPv4 server 4, sends the message Sthereto, and after that, waits for a response for a predetermined time.The IP transmitting and receiving means 10 transmits the message S tothe DHCPv4 server 4 via the IPv4 network.

[0171] The DHCPv4 server 4 which received the message S gives the IPv4address to the IPv6 address (2 : : 1) of the IPv6 terminal 115 (it isnow assumed that “133. 144. 96. 1” is given) and returns an IPv4 addressgiven message (hereinafter, called a ‘message T’).

[0172] The IP transmitting and receiving means 10 which received themessage T sends the message T to the IPv4 address capturing means 14.The IPv4 address capturing means 14 obtains the IPv4 address (133. 144.96. 1) from the message T and notifies the DNS substituting means 13.The DNS substituting means 13 forms the message U to notify of the IPv4address (133. 144. 96. 1) as an IP address corresponding to the domainname of the IPv6 terminal 115 and sends it to the IP transmitting andreceiving means 10. The IP transmitting and receiving means 10 transmitsthe message U to the IPv4-IPv6 converting apparatus 1 via the IPv4network. The DNS substituting means 13 sends the IPv4 address (133. 144.96. 1) captured from the DHCPv4 server 4 to the IP address conversioninformation holding means 15 in correspondence to the IPv6 address (2 :: 1) of the IPv6 terminal 115. The IP address conversion informationholding means 15 forms a new entry into the IP address conversion table151 and registers a correspondence of the IPv4 address (133. 144. 96. 1)and the IPv6 address (2 : : 1) of the IPv6 terminal 115. The entry inthe IP address conversion table 151 is deleted when it is not referredto for a predetermined time. The IPv4 address (133. 144. 96. 1) isreturned to the DHCPv4 server 4.

[0173] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 1 which received the response from the IPv4-IPv6converting apparatus 111 within the predetermined time sends thereceived message U to the DNS substituting means 13. The DNSsubstituting means 13 obtains the IPv4 address (133. 144. 96. 1)corresponding to the domain name of the IPv6 terminal 115 from themessage U. The DNS substituting means 13 forms a message (hereinafter,called a ‘message V’) to notify of an ‘IPv4-mapped IPv6 address’ (: :FFFF : 133. 144. 96. 1) in which the IPv4 address (133. 144. 96. 1)derived from the message U has been set as an IP address correspondingto the domain name of the IPv6 terminal 115, instructs the IPtransmitting and receiving means 10 so as to transfer the message V tothe IPv6 terminal 5, and sends the message V thereto. The IPtransmitting and receiving means 10 transmits the message V to the IPv6terminal 5 via the IPv6 network.

[0174] As shown in FIG. 21, the IPv6 terminal 5 transmits the IPv6packet in which the IPv6 address (: : FFFF : 133. 144. 96. 1) notifiedfrom the Ipv4-Ipv6 converting apparatus 1 has been set into the IPv6destination address field of the IPv6 packet and the own IPv6 address (1: : 1) has been set into the IPv6 source address field to the IPv4-IPv6converting apparatus 1 through the IPv6 network.

[0175] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 1 sends the received IPv6 packet to the IP headerconverting means 11. The IP header converting means 11 extracts the IPv6address ( : : FFFF : 133. 144. 96. 1) from the IPv6 destination addressfield of the IPv6 packet, converts the IPv6 address to the IPv4 address(133. 144. 96. 1) of 32 bits by deleting the fixed pattern of 96 bitsmentioned above, and sets the IPv4 address into the IPv4 destinationaddress field of the IPv4 packet. The IP header converting means 11extracts the IPv6 address (1 : : 1) from the IPv6 source address fieldof the IPv6 packet and sends it to the IP address conversion informationholding means 15.

[0176] The IP address conversion information holding means 15 retrievesthe IP address conversion table 151 by using the IPv6 address (1 : : 1)of the IPv6 terminal 5 as a key and notifies the IP header convertingmeans 11 of a message indicating whether the relevant entry has beenregistered or not.

[0177] The IP header converting means 11 which received the notificationthat the relevant entry is not registered yet in the IPv6 address (1 :: 1) of the IPv6 terminal 5 instructs the IPv4 address capturing means14 so as to capture the IPv4 address and, after that, waits for aresponse for a predetermined time. The IPv4 address capturing means 14forms an IPv4 address capturing message (hereinafter, called a ‘messageW’), instructs the IP transmitting and receiving means 10 so as totransfer the message W to the DHCPv4 server 4, sends the message Wthereto, and after that, waits for a response for a predetermined time.The IP transmitting and receiving means 10 transmits the message W tothe DHCPv4 server 4 via the IPv4 network.

[0178] The DHCPv4 server 4 which received the message W gives the IPv4address to the IPv6 address (1 : : 1) of the IPv6 terminal 5 (it is nowassumed that “133. 144. 95. 1” is given) and returns an IPv4 addressgiven message (hereinafter, called a ‘message X’).

[0179] The IP transmitting and receiving means 10 which received themessage X sends the message X to the IPv4 address capturing means 14.The IPv4 address capturing means 14 obtains the IPv4 address (133. 144.95. 1) from the message X and notifies the IP header converting means11. The IP header converting means 11 sends the IPv4 address (133. 144.95. 1) captured from the DHCPv4 server 4 to the IP address conversioninformation holding means 15 in correspondence to the IPv6 address (1 :: 1) of the IPv6 terminal 5. The IP address conversion informationholding means 15 forms a new entry into the IP address conversion table151 and registers a correspondence of the IPv4 address (133. 144. 95. 1)and the IPv6 address (1 : : 1) of the IPv6 terminal 5. The entry in theIP address conversion table 151 is deleted when it is not referred tofor a predetermined time. The IPv4 address (133. 144. 95. 1) is returnedto the DHCPv4 server 4.

[0180] When a notification that the relevant entry has been registeredin the Ipv6 address (1 : : 1) of the IPv6 terminal 5 is received or whenthe registration of the relevant entry into the IPv6 address (1 : : 1)of the IPv6 terminal 5 is completed, the IP header converting means 11sets the IPv4 address (133. 144. 95. 1) into the IPv4 source addressfield of the IPv4 packet. The IPv4 packet formed by performing the IPheader conversion as mentioned above is sent to the IP transmitting andreceiving means 10. The IP transmitting and receiving means 10 transmitsthe IPv4 packet to the IPv4-IPv6 converting apparatus 111 through theIPv4 network.

[0181] The IP transmitting and receiving means 10 of the IPv4-IPv6converting apparatus 111 sends the received IPv4 packet to the IP headerconverting means 11. The IP header converting means 11 extracts the IPv4address (133. 144. 95. 1) from the IPv4 source address field of the IPv4packet, converts the IPv4 address to an ‘IPv4-mapped IPv6 address’ (: :FFFF : 133. 144. 95. 1) of 128 bits by adding the fixed pattern of 96bits mentioned above, and sets it into the IPv6 source address field ofthe IPv6 packet. The IP header converting means 11 extracts the IPv4address (133. 144. 96. 1) from the IPv4 destination address field of theIPv4 packet and sends the IPv4 address to the IP address conversioninformation holding means 15. The IP address conversion informationholding means 15 obtains the IPv6 address (2 : : 1) corresponding to theIPv4 address (133. 144. 96. 1) with reference to the IP addressconversion table 151 and sends the IPv6 address to the IP headerconverting means 11. The IP header converting means 11 sets the IPv6address (2 : : 1) into the IPv6 destination address field of the IPv6packet. The IPv6 packet formed by performing the IP header conversion asmentioned above is sent to the IP transmitting and receiving means 10.The IP transmitting and receiving means 10 transmits the IPv6 packet tothe IPv6 terminal 115 through the IPv6 network.

[0182] In this manner, the communication from the IPv6 terminal 5 to theIPv6 terminal 115 can be started.

[0183] Although the invention has been described above by using thecommunication between the two IPv6 terminals when there is the IPv4network on the communication path, a similar construction can be alsoapplied to a communication between the two IPv4 terminals when there isthe IPv6 network on the communication path.

[0184] Although the invention has been also described above by using thecommunication between the two IPv6 terminals when there is only one IPv4network on the communication path, a similar construction can be alsoapplied to a communication between the two IPv6 terminals when there area plurality of IPv4 networks and IPv6 networks on the communicationpath, a communication between the two IPv4 terminals, and acommunication between the IPv4 terminal and the IPv6 terminal.

[0185] According to the IPv4-IPv6 communicating method and IPv4-IPv6converting apparatus of the invention, only the IPv6 protocol isinstalled to the IPv6 terminal and even if the IPv4 address is notpreviously fixedly allocated, the communication between the IPv4terminal and the IPv6 terminal can be executed. Further, since thepartner can be designated not by the actual IP address but by the domainname, the communication can be performed without mutually being awarewhether the partner uses the IPv4 or IPv6 as a communication protocol.

[0186] As for the registration and deletion of the entry in theconversion table of the IPv4 address and IPv6 address, by limiting bythe setting of a manager, the system can also become a fire wall betweenthe IPv4 network and the IPv6 network.

[0187] A communication between the two IPv6 terminals partitioned by theIPv4 network can be also executed without preliminarily performing anyspecial setting. Similarly, a communication between the two IPv4terminals partitioned by the IPv6 network can be executed withoutpreliminarily performing any special setting.

[0188] Further, even if any special setting is not preliminarilyperformed, a communication between the IPv4 terminals partitioned by aplurality of IPv4 networks and IPv6 networks, a communication betweenthe IPv6 terminals, and a communication between the IPv4 terminal andthe IPv6 terminal can be executed.

1. A method of communicating an IPv4 terminal of an IPv4 network and anIPv6 terminal of an IPv6 network in a communication network systemcomprising said IPv4 network, said IPv6 network, and an IPv4-IPv6converting apparatus connected to both of said networks, comprising thesteps of: in accordance with that said IPv4 terminal inquires an IPaddress for a domain name of said IPv6 terminal to said IPv4-IPv6converting apparatus, obtaining, in said IPv4-IPv6 converting apparatus,an IPv6 address for said domain name from a DNSv6 server apparatus formanaging domain information of said IPv6 network; capturing, in saidIPv4-IPv6 converting apparatus, an IPv4 address corresponding to saidIPv6 address from a DHCPv4 server apparatus for dynamically allocatingIPv4 addresses and notifying said IPv4 terminal; forming, in said IPv4terminal, a first IPv4 packet by setting an own IPv4 address to an IPv4source address and by setting said notified IPv4 address to an IPv4destination address, and sending said first IPv4 packet to saidIPv4-IPv6 converting apparatus; and converting, in said IPv4-IPv6converting apparatus, the IPv4 source address included in said receivedfirst IPv4 packet to an IPv6 source address by adding fixed data to saidIPv4 source address, converting an IPv4 destination address included insaid IPv4 packet to a corresponding IPv6 address, forming a first IPv6packet from said first IPv4 packet, and sending said first IPv6 packetto said IPv6 network.
 2. A method according to claim 1, furthercomprising the steps of: forming, in said IPv6 terminal, a second IPv6packet by setting an own IPv6 address to the IPv6 source address and bysetting said IPv6 source address included in said received first IPv6packet to an IPv6 destination address, and sending said second IPv6packet to said IPv4-IPv6 converting apparatus; and converting, in saidIPv4-IPv6 converting apparatus, the IPv6 destination address included insaid received second IPv6 packet to the IPv4 destination address bydeleting the fixed data from said IPv6 destination address, convertingthe IPv6 source address included in said IPv6 packet to thecorresponding IPv4 address, forming a second IPv4 packet from saidsecond IPv6 packet, and sending said second IPv4 packet to said IPv4network.
 3. A method of communicating an IPv4 terminal of an IPv4network and an IPv6 terminal of an IPv6 network in a communicationnetwork system comprising said IPv4 network, said IPv6 network, and anIPv4-IPv6 converting apparatus connected to both of said networks,comprising the steps of: in accordance with that said IPv6 terminalinquires an IP address for a domain name of said IPv4 terminal to saidIPv4-IPv6 converting apparatus, obtaining, in said IPv4-IPv6 convertingapparatus, an IPv4 address for said domain name from a DNSv4 serverapparatus for managing domain information of said IPv4 network, andnotifying said IPv6 terminal of a*n IPv6 address derived by adding fixeddata to said IPv4 address; forming, in said IPv6 terminal, a first IPv6packet by setting an own IPv6 address to an IPv6 source address and bysetting said notified IPv6 address to an IPv6 destination address, andsending said first IPv6 packet to said IPv4-IPv6 converting apparatus;and converting, in said IPv4-IPv6 converting apparatus, said IPv6destination address included in said received first IPv6 packet to anIPv4 destination address by deleting fixed data from said IPv6destination address, capturing dynamically an IPv4 address correspondingto the IPv6 source address included in said first IPv6 packet from aDHCPv4 server apparatus for dynamically allocating the IPv4 address,forming a first IPv4 packet from said first IPv6 packet by using saidIPv4 address, and sending said first IPv4 packet to said IPv4 network.4. A method according to claim 3, further comprising the steps of:forming, in said IPv4 terminal, a second IPv4 packet by setting an ownIPv4 address to the IPv4 source address and by setting said IPv4 sourceaddress included in said received first IPv4 packet to an IPv4destination address, and sending said second IPv4 packet to saidIPv4-IPv6 converting apparatus; and converting, in said IPv4-IPv6converting apparatus, the IPv4 source address included in said receivedsecond IPv4 packet to the IPv6 source address by adding fixed data tosaid IPv4 source address, converting the IPv4 destination addressincluded in said IPv4 packet to a corresponding IPv6 address, forming asecond IPv6 packet from said second IPv4 packet, and sending said secondIPv6 packet to said IPv6 network.
 5. An IPv4-IPv6 converting apparatusconnected to an IPv4 network and an IPv6 network, comprising: (a) IPv4address capturing means for capturing an IPv4 address from a DHCPv4server of said IPv4 network; (b) IP address conversion informationholding means for holding an IPv6 address of said IPv6 terminal and saidcaptured IPv4 address in correspondence to each other; (c) IPtransmitting and receiving means for transmitting and receiving an IPv4packet to/from said IPv4 network and for transmitting and receiving anIPv6 packet to/from said IPv6 network; (d) IP header converting meansfor converting an IPv4 source address included in said received IPv4packet to an IPv6 source address by adding fixed data to said IPv4source address, for converting an IPv4 destination address included insaid IPv4 packet to a corresponding IPv6 destination address by usingsaid IP address conversion information holding means, and for forming acorresponding IPv6 packet from said IPv4 packet; and (e) DNSsubstituting means for receiving an inquiry of an IP address for adomain name of the IPv6 terminal from the IPv4 terminal, for obtainingan IPv6 address by inquiring to the DNSv6 server, for notifying saidIPv4 terminal of said IPv4 address if said IPv4 address corresponding tosaid IPv6 address has been held in said IP address conversioninformation holding means, for capturing the IPv4 address by said IPv4address capturing means if the IPv4 address corresponding to said IPv6address is not held in said IP address conversion information holdingmeans, for holding said IPv6 address and said captured IPv4 address intosaid IP address conversion information holding means so as to correspondto each other, for notifying said IPv4 terminal of said IPv4 address,similarly for receiving an inquiry of an IP address for a domain name ofthe IPv4 terminal from the IPv6 terminal, for obtaining an IPv4 addressby inquiring to the DNSv4 server, for notifying said IPv6 terminal ofsaid IPv6 address if the IPv6 address corresponding to said IPv4 addresshas been held in said IP address conversion information holding means,for capturing the IPv6 address by said IPv6 address capturing means ifthe IPv6 address corresponding to said IPv4 address is not held in saidIP address conversion information holding means, for holding said IPv4address and said captured IPv6 address into said IP address conversioninformation holding means so as to correspond to each other, and fornotifying said IPv6 terminal of said IPv6 address.
 6. An apparatusaccording to claim 5, further comprising IPv6 address means forcapturing an IPv6 address from a DHCPv6 server of said IPv6 network. 7.An apparatus according to claim 5, wherein said IP address conversioninformation holding means deletes an address which is not referred tofor a predetermined time in the correspondence of said IPv6 address andIPv4 address held and returns the deleted IPv4 address to said DHCPv4server of said IPv4 network.
 8. An apparatus according to claim 5,further comprising IPv4 address pool means for pooling a plurality ofIPv4 addresses, and wherein said IP address conversion informationholding means captures said plurality of IPv4 addresses from said DHCPv4server of said IPv4 network at the time of activation of the apparatus,pools said captured IPv4 addresses into said IPv4 address pool means,and captures the IPv4 address corresponding to the IPv6 address of saidIPv6 terminal from said IPv4 address pool means.
 9. An apparatusaccording to claim 5, further comprising DHCPv4 server means fordynamically allocating the IPv4 address, and wherein said DHCPv4 servermeans is used in place of said DHCPv4 server of said IPv4 network.